Eels
by John Walsby
Most of our native
freshwater fish - such as inanga, kokopu, koaro and mudfish -
are quite small and little known because compared with introduced
trout and salmon, they are tricky to catch, are not good game
fish and offer too little for the table.
The juveniles of these
fish, especially of inanga, were once very well known. As whitebait
returning from the sea to spend their adult lives in freshwater,
they were caught in millions ending up in fritters or as snacks
on cocktail biscuits.
Today they are less
common because there are far fewer adults. It is likely that this
is because of changes to our waterways. Many of the wetlands around
little creeks and side streams have been reclaimed or had the
shady bush removed from their banks. Elsewhere waterways have
been polluted with silt and chemicals. The silt is washed down
from bare land where forests have been clear felled or where there
is crop farming or residential and industrial development. It
clogs the gills of small animals and also restricts the growth
of microscopic plant plankton by clouding the water and cutting
out daylight.
Eels are by far our
largest native freshwater fish and seem more tolerant of murky
waters than other native species. They are therefore often associated
with mud holes and dank backwater seepages but like other freshwater
fish they like and need oxygenated water that is free of pollutants.
They are extremely
long bodied fish with just one pair of lateral fins and have narrow
dorsal and ventral fins that join as a continuous tape around
the tip of the tail. Of the 16 different species found in the
world, only two, the longfinned and shortfinned eels, are native
to New Zealand. The longfinned is only found in this country but
shortfinned eels also live in Australia and other islands of the
South Pacific.
The long finned eel
is distinguished by the dorsal fin extending much farther forward
than the ventral one and it has a larger mouth. The dorsal fin
in the short finned eel starts just in front of the ventral.
Eels live from 12 to
more than 50 years depending on their species and mature adults
are sometimes very large. Shortfinned eels grow up to a metre
long and weigh about 3á5kg, though most are a little smaller
when they return to the sea to breed after 12 to 15 years living
in freshwater.
The longfinned eel
is one of the world's largest. Today after decades of fairly intensive
fishing the very large specimens that were once caught are rare.
An eel measuring 1á25m long and weighing 10kg would be
considered large now but in the past, 15 kg, 1á5m long
eels were not exceptional considering that a few eels 2m long
and weighing up to 50kg have been caught. Eels this large are
probably well over 50 years old.
An eels autumn migration
back to the sea to breed is its last journey. It stops feeding,
becomes darker and undergoes a number of body changes. Its bulbous
head becomes flattened and pointed, its eyes almost double in
size and its pectoral fins become larger.
They breed in deep
water far away from land, often towards the tropics. There each
female may lay a couple of million eggs from which small leaf-shaped
larval fish hatch. Swimming and drifting in ocean currents it
probably takes these larvae over a year to return to New Zealand
coastal waters where they change quickly to long slender fish
but remain almost transparent.
These little finger
length juveniles, called glass eels, move into our estuaries in
the spring. There over about a fortnight, they undergo internal
bodily changes and develop into pigmented young eels, called elvers.
The elvers remain in the estuaries for a while, then, moving mostly
at night, they steadily migrate upstream. After a year or more
they eventually arrive at the upper reaches of streams and rivers
and at some inland lakes, negotiating rapids, natural waterfalls
and even the spillways of some hydroelectric dams on the way.
Young eels feed mainly
on insect larvae, small snails worms and midges. Only when they
are quite large, many years later, do they start to feed on larger
prey such as koura and small fish. Very large eels are also known
to take ducklings and are very sensitive to the smell of carrion
upstream, a fact that eel fishermen exploit when baiting their
traps with blood and offal.
